This invention relates to a semiconductor memory device using a ferroelectric material.
From the point of view of write-in and read-out, memory devices may be divided into the RAM (random access memory) type which allows data to be written in and read out from any address and the ROM (read only memory) type which allows data only to be read therefrom. In other words, data can be written into a RAM and can be read therefrom and for this reason, the speeds of reading out and writing in data are usually set about equal to each other. As for a ROM, data can be read therefrom whenever they are needed but they cannot be written in, or may be written in but the speed of writing in data is set very much slower than the speed of reading them out. For this reason, a RAM is usually used for storing data which must be updated frequently, while a ROM is usually used for storing a program which does not have to be rewritten frequently.
Examples of a magnetic memory device include disk memories such as hard disks and floppy disks. Although they may be classified as a RAM because data can be freely written in and read out, they can also function as a ROM if a write-inhibiting means is provided so as to be used only for reading out data. In other words, a single memory device can sometimes be used both as a RAM and a ROM. As for optical memory devices such as CD-ROMs, they are used only for reading out data.
As for semiconductor memory devices, DRAMs and SRAMs may be cited as examples of RAMs while examples of ROM include mask ROMs, fuse-type bipolar PROMs (programmable read only memories) and diode destructible PROMs. Examples of a non-volatile semiconductor device include EPROMs (erasable PROMs) and EEPROMs (electrically erasable PROMs). DRAMs and SRAMs are not provided with any non-volatile characteristics like ROMs and cannot physically prevent write-in. On the other hand, aforementioned kinds of semiconductor ROMs cannot allow data to be freely written in and hence cannot serve as a RAM. In summary, unlike disk memories as an example of magnetic memory devices, there has not been developed yet a technology for making a semiconductor memory device to serve both as a RAM and, with the help of a write-inhibiting means, also as a ROM.
Recently, ferroelectric memory devices are being developed as an example semiconductor memory devices. Ferroelectric memories function to store data by means of residual polarization of a ferroelectric material and not only have non-volatility but also are capable of having data written in and read out at a high speed like DRAMs. In other words, ferroelectric memories can function as RAMs but they cannot serve as a ROM since they cannot inhibit write-in.
One of the technological problems involved in the development of ferroelectric memory devices has related to the so-called "imprint characteristic." This is the problem, for example, of the phenomenon that, if a data item such as "0" is stored in a memory cell for a long time such as several years and if an attempt is made thereafter to write "1" in this memory cell, the cell will find it difficult to store "1" but will return to the condition of storing "0." It is known that this phenomenon occurs when voltage pulses of a same polarity are successively applied or when heat is applied under a polarized condition. For this reason, it has been a common practice to apply voltage pulses continuously or heat in order to test the imprint characteristic by means of such a load.
Such an "imprint condition" at which it becomes difficult to write in a data item into a memory cell is now understood to be caused when the hysteresis characteristic of the ferroelectric material is deformed or shifted. FIGS. 8A, 8B and 8C show how the hysteresis curve becomes deformed from an initial condition shown in FIG. 8A to FIG. 8B and then to FIG. 8C.
When the hysteresis curve was as shown in FIG. 8A, for example, let us assume, as described in Japanese Patent Publication Tokkai 8-36888, that two voltage levels V.sub.1 and V.sub.2 are taken corresponding to a positive polarization condition and a negative polarization condition in order to distinguish data "0" and "1" and that a reference voltage V.sub.ref (not shown) is set between V.sub.1 and V.sub.2 such that a detected voltage V.sub.d at the time of detection is compared with this reference voltage V.sub.ref and the stored data item is identified as "1" or "0" respectively if V.sub.d &gt;V.sub.ref and V.sub.d &lt;V.sub.ref.
As the hysteresis curve becomes deformed, as shown in FIGS. 8B and 8C, however, the difference between V.sub.1 and V.sub.2 becomes smaller and their size relationship may even become reversed, as shown in FIG. 8C. In such a situation, since voltage V.sub.1 corresponding to data item "1" is smaller than the reference voltage V.sub.ref, even if it is attempted to write "1" where it was "0", the stored data item may not necessarily be recognized as "0". In other words, the memory cell seems to go back to the initial condition before the rewriting is effected, and this is the so-called imprint condition.
It has indeed been ascertained that the so-called imprint condition does not mean that the memory condition of a memory device has irreversibly affixed but that the memory device goes back to its initial condition even if voltage pulses of the polarity corresponding to a data item opposite to the stored data item are repeated applied.
The aforementioned imprint characteristic has been considered as a kind of deterioration in the characteristics of a memory device. From a different point of view, however, this property that the memory condition is kept unchanged as before even if an attempt is made to write in a new data item may be considered to be similar to the function of inhibiting write-in, say, into a disk memory of a magnetic memory device, as explained above.
It is therefore an object of this invention to affirmatively make use of the imprint characteristic in a memory device such that a semiconductor memory device, which already has the functions of a RAM, can also function as a ROM.